1. Field of the Invention
The present invention relates to a magnetic bearing, and more particularly, to a magnetic bearing of a drive motor which drives an optical deflector used in an electrophotographic device such as a laser printer, digital copying machine, and the like.
2. Description of the Related Art
Normally, an image forming device includes an optical deflector 60 shown in FIG. 10 as means for scanning an optical beam such as a laser and the like. The optical deflector 60 is formed by a rotary polygon mirror 12, in which a plurality of reflecting mirrors are formed at the outer periphery, and a dynamic pressure air bearing type drive motor 62 which rotates rapidly the rotary polygon mirror 12 (see Japanese Patent Application Laid-Open (JP-A) No. 60-55316, Japanese Patent Application Laid-Open (JP-A) No. 60-244913, and Japanese Patent Application Laid-Open (JP-A) No. 62-85216).
The drive motor 62 includes a stationary shaft 66, at which grooves 64 for generating dynamic pressure are provided, and a sleeve 68, which is rotatably provided at the outer peripheral surface of the stationary shaft 66 with a predetermined gap therebetween. The drive motor 62 has a so-called radial dynamic pressure air bearing mechanism, i.e., the sleeve 68 rotates without abutting the stationary shaft 66 due to the dynamic pressure effect generated at the time of rotation.
Further, the supporting structure of the sleeve 68 in the axial direction thereof is the one in which the sleeve 68 is levitated by magnetic attracting force generated between a ring-shaped permanent magnet 72 for lift (hereinafter, "levitational permanent magnet") and a stationary permanent magnet 74. The levitational permanent magnet 72 is mounted to a pedestal 70, on which the rotary polygon mirror 12 is disposed, outwardly in the radial direction thereof, and the stationary permanent magnet 74 is disposed at the outer peripheral portion of the levitational permanent magnet 72.
On the other hand, a magnetic circuit which is required for the rotation of the sleeve 68 is formed between a yoke 78 and a permanent magnet 76 for rotation (hereinafter, "rotational permanent magnet") which is fixed to the outer peripheral portion of the pedestal 70. Moreover, as a plurality of magnetic poles of the rotational permanent magnet 76 are detected by a magnetic detection element 80 and a drive coil 84 provided on a circuit board 82 is energized at a predetermined timing logic, the torque of the sleeve 68 is obtained.
However, in the drive motor 62, because the levitational permanent magnet 72 and the stationary permanent magnet 74 are provided outwardly in the radial direction of the sleeve 68, a housing that accommodates the components projects outwardly. As a result, the drive motor 62 becomes large.
Further, because the magnetic circuit for rotational drive and the magnetic circuit for magnetic bearing are formed separately, the number of components cannot be reduced. Furthermore, since the borderline region of the magnetic poles of the stationary permanent magnet 74 at a fixed side and that of the levitational permanent magnet 72 at a rotational side face to each other, when one of the permanent magnets is formed thinner than the other, a portion in which magnetic force is small is generated and the magnetic attracting force may be thereby decreased.